Increasing use of wireless and high frequency technology in consumer products has resulted in more radiated signals in portions of the spectrum available for such products. Noise of various kinds can be injected into integrated circuits, thus reducing performance. One instance of such noise is beats—interference between two oscillators operating at similar frequencies. Another instance of such noise is harmonics—energy at a frequency related to an oscillator frequency produced due to non-ideal (real-world) aspects of circuit components. Both of these instances of noise are preferably reduced in circuits.
Preferably, blocking or reducing noise injected into circuits and systems is achieved in an inexpensive manner. Noise may be injected due to adjacent channels, image tones, jamming or blocking signals, and for other reasons. However, blocking or reducing noise is not the primary function of a system, it is a necessary addition allowing the rest of the system to perform its intended function, such as receiving encoded television or audio signals for example.
Shown in FIG. 1A is a block diagram illustrating the mixing or converting of a signal by a periodic signal. As shown, a signal, x(t)=f(t)cos(ω1t), is provided at input 102.
As further shown in FIG. 1A, a periodic signal, z(t)=2 cos(ω1+ω2)t, is provided at input 106 to mixer 104. Note that the frequency, ω1, is a shifting frequency that for the purposes of the present discussion can be a negative value. The Fourier transform, Z(s), of the periodic signal, z(t), is written as
      Z    ⁡          (      s      )        =                              2          ⁢          π                                      ω            1                    +                      ω            2                              ⁢              Π        ⁡                  (                                    π                                                ω                  1                                +                                  ω                  2                                                      ⁢            s                    )                      =                  π                              ω            1                    +                      ω            2                              ⁡              [                              δ            ⁡                          (                                                                    π                                                                  ω                        1                                            +                                              ω                        2                                                                              ⁢                  s                                +                                  1                  2                                            )                                +                      δ            ⁡                          (                                                                    π                                                                  ω                        1                                            +                                              ω                        2                                                                              ⁢                  s                                -                                  1                  2                                            )                                      ]                            which can be rewritten as        
      Z    ⁡          (      s      )        =            δ      ⁡              (                  s          +                                                    ω                1                            +                              ω                2                                                    2              ⁢              π                                      )              +                  δ        ⁡                  (                      s            -                                                            ω                  1                                +                                  ω                  2                                                            2                ⁢                π                                              )                    .      
Both the signal, x(t), and the periodic signal, z(t), are then provided to the inputs of mixer 104. The resulting signal, e(t), at output 108 is therefore the product, e(t)=x(t)z(t) whose Fourier transform is the convolution, E(s)=X(s)*Z(s). From the above results for X(s) and Z(s) and after some mathematical manipulation, we have:
                              E          ⁡                      (            s            )                          =                                            X              ⁡                              (                s                )                                      *                          δ              ⁡                              (                                  s                  +                                                                                    ω                        1                                            +                                              ω                        2                                                                                    2                      ⁢                      π                                                                      )                                              +                      δ            ⁡                          (                              s                -                                                                            ω                      1                                        +                                          ω                      2                                                                            2                    ⁢                    π                                                              )                                                                        E          ⁡                      (            s            )                          =                              X            ⁡                          (                              s                +                                                                            ω                      1                                        +                                          ω                      2                                                                            2                    ⁢                    π                                                              )                                +                      X            ⁡                          (                              s                -                                                                            ω                      1                                        +                                          ω                      2                                                                            2                    ⁢                    π                                                              )                                                                        E          ⁡                      (            s            )                          =                                            1              2                        ⁢                          F              ⁡                              (                                  s                  +                                                                                    2                        ⁢                                                  ω                          1                                                                    +                                              ω                        2                                                                                    2                      ⁢                      π                                                                      )                                              +                                    1              2                        ⁢                                          F                ⁡                                  (                                      s                    -                                                                  ω                        2                                                                    2                        ⁢                        π                                                                              )                                            .                                          
Thus, in the time domain, we have the inverse Fourier transform, e(t), ase(t)=f(t)cos((2ω1+ω2)t)+f(t)cos(ω2t)
Note that the first term, f(t)cos((2ω1+ω2)t), is undesired and is therefore filtered out using filter 110. The output 112 of filter 110 is thereforee′(t)=f(t)cos(ω2t).
Thus, the filtered output signal, e′(t), is a shifted version of the input signal, x(t).
Mixer 100 of FIG. 1A can be used as part of a television tuner including single-conversion and dual-conversion television tuners for example. More recently, dual conversion tuners find wide application in television and cable tuners. In a dual conversion television tuner, as the name implies, two conversions (i.e., two mixers) are implemented. Moreover, in a dual conversion television tuner, the input frequency, ω1+2ω2, to bandpass filter 110 is referred to as an image frequency of the desired frequency, ω1. The image frequency can sometimes be a problem and therefore may be filtered in such television tuner applications.
In a dual-conversion architecture 150, such as that shown in FIG. 1B, a first mixer 104 is implemented that up-converts a received RF signal, x(t), at input 102 to a signal, e(t), at mixer output 108 having a first intermediate frequency that is then filtered by filter 110 (note that like-numbered components between FIG. 1A and FIG. 1B operate similarly). The filtered signal, e′(t), at input 112 to mixer 114 is then at a nominal amplitude while all the rejected signals are at a much lower amplitude. The filtered signal, e′(t), is then input to a second mixer 114 where e′(t) is mixed with signal y(t) at input 116 to down-convert the filtered signal, e′(t), into a desired IF signal, m(t), at output 118. The desired IF signal, m(t), is the channel a user wishes to view. In actual implementation, amplification of signals and other factors need to be considered.
Using the techniques described above, a prior art tuner 200 is built as shown in FIG. 2. Tuner 200 includes dual mixers 202 and 204 which are similar in operation to mixers 104 and 114 described with reference to FIG. 1B. Dual mixers 202 and 204 receive local oscillator signals from local oscillators LO1 264 and LO2 268 at mixer inputs 206 and 208, respectively. Oscillator circuit 210 drives local oscillators LO1 264 and LO2 268 responsive to external clock signal 212 generated by crystal 214. As shown in FIG. 2, crystal 214 generates external clock signal 212 with a frequency of 5.25 MHz. External clock signal 212 is then directed to reference frequency generator 240 which generates a reference signal for phased lock loop PLL1 220 of oscillator circuit 210 which provides input signal 262 to local oscillator LO1 264. External clock signal 212 is further directed to reference frequency generator 216. Using external clock signal 212 with a frequency of 5.25 MHz, reference frequency generator generates reference signal 218 operating at 2.625 MHz. Reference signal 218 is then directed to oscillator circuit 210 and, more particularly, to phase locked loop PLL2 228. Using the reference signal 218, phase locked loop PLL2 228 is configured to provide signal 266 to local oscillator LO2 268.
Tuner 200 can be configured or implemented to operate in television systems where, for example, signals representing individual channels are assigned to specific frequencies in a defined frequency band. Illustratively, in the United States, television signals are generally transmitted in a band from 55 MHz to 806 MHz. Such a radio frequency (RF) signal can, therefore, be received at input 230 of tuner 200. The received RF signal passes through a front-end filter 232 that filters out any signals outside of the frequency range of interest (i.e., outside of 55 MHz to 806 MHz). Filter 232 can be a bandpass filter or, more typically, a low pass filter that is designed to remove all frequencies above an input cutoff frequency, ωc. The input cutoff frequency, ωc, is chosen to be higher than the frequencies of the channels in the television band. The output 234 of filter 232 is then directed to amplifier 236 to provide the signal (238) that is then directed to mixer input 238 of mixer 202. As described above, mixer 202 also receives at input 206 a local oscillator signal from local oscillator LO1 264.
The output 242 of mixer 202 is a first intermediate frequency signal IF1 which is directed to IF filter 244. Typically, the frequency of local oscillator LO1 264 is variable and selected responsive to a desired channel in the RF signal at input 230. Moreover, the frequency local oscillator LO1 264 is selected such that mixing of the local oscillator signal at input 206 and the filtered and amplified signal at mixer input 238 generates signal IF1 at a specified frequency or within a narrow range of frequencies as may be desired. ° F. filter 244 is a band pass filter that is used to remove unwanted frequencies and spurious signals from the signal IF1. The output of ° F. filter 244 is then directed to mixer input 246 of mixer 204 which also receives a second local oscillator signal at input 208 that is generated by local oscillator LO2 268. Mixer 204 mixes these signals to generate a second intermediate frequency signal, IF2, which is then directed to input 248 of amplifier 250. In television tuner applications, mixer 204 may be an image rejection mixer that rejects image frequencies from the second intermediate frequency signal, IF2.
Local oscillator signal LO2 268 can be implemented to generate a variable or fixed frequency signal depending upon whether the first intermediate frequency signal, IF1, is at a fixed frequency or if it varies over a range of frequencies. Regardless, the frequency of the signal generated by local oscillator LO2 268 is selected to generate a fixed frequency signal IF2 that is directed to input 248 of amplifier 250. Output 252 of amplifier 250 is, therefore, amplified signal IF2′ that is directed to additional processing circuitry 254 to generate either digital or analog television signals as may be desired. Further included in tuner 200 is serial control circuitry 256. Among other things, serial control circuitry 256 controls phased locked loops PLL1 and PLL2 to set the frequencies of local oscillators LO1 264 and LO2 268.
Advances have been made to integrate much circuitry into a single integrated circuit. Prior art applications typically integrate the circuitry enclosed by box 258. Notably, front end filter 232 and IF filter 244 are typically placed outside of an integrated circuit. This is not surprising because of the size and other design considerations of such filters. The present invention, however, teaches a manner for integrating IF filter 244 onto a tuner IC.